1. Field of the Invention
This invention relates to a process for the manufacture of multilayer printed circuit boards. In particular, the invention relates to reducing or eliminating localized delamination of such a printed circuit board during drilling of through-holes.
2. Description of the Prior Art
Multilayer printed circuit boards are used for a variety of electrical applications and provide notable advantages of conservation of weight and space. A multilayer board is comprised of two or more circuit layers, each circuit layer separated from another by one or more layers of dielectric material. Circuit layers are formed by applying a copper layer onto a polymeric substrate. Printed circuits are then formed on the copper layers by techniques well known in the art, for example print and etch to define and produce the circuit traces.
After lamination, the multiple circuit layers are connected by drilling through-holes through the board surface. Resin smear from through-hole drilling is removed under rather stringent conditions, for example treatment with concentrated sulfuric acid or hot alkaline permanganate, and then the through-holes are further processed and plated to provide a conductive interconnecting surface.
Prior to lamination, the circuit layers are typically treated with an adhesion promoter to improve bond strength between each circuit layer and the interleaving resin layers. One favored method of improving such bond strength is oxidative treatment of a circuit layer to form a copper oxide surface coating thereon.
Persistent problems exist however with the use of a copper oxide adhesion promoter. In particular, leaching of the copper oxide coating occurs laterally along through-hole walls during the through-hole processing and plating steps of board manufacture, a phenomenon known in the art as "pink ring". Once the copper oxide is leached, pinkish copper metal is exposed thus giving rise to the term pink ring. While not necessarily a cause for board rejection in itself, pink ring signals a flaw in the board construction, namely a weak or fractured circuit layer/resin bond.
Such fracturing of board layers traditionally was believed to result from the harsh conditions of through-hole processing steps, for example by direct lateral attack of desmear solutions and electroplating baths. However, more recent views hold that such board delamination occurs earlier in the manufacturing process during through-hole drilling. See, for example, Graham, et al., Circuit World, vol. 16, no. 4, pp. 4-12 (1990), incorporated herein by reference. Localized fractures and delamination occur along the oxided inner layer/resin interface from the high mechanical stresses generated during the drilling process. During subsequent through-hole treatment steps, aggressive chemicals seep along these fractures dissolving portions of the copper oxide layer and thereby expose copper metal.
Pink ring has been addressed by forming a conversion coating on a copper oxide surface layer where the conversion coating is resistant to chemical attack during subsequent manufacturing steps. See, for example, U.S. Pat. No. 4,775,444, incorporated herein by reference. This approach, however, does not eliminate pink ring as known conversion coatings are not wholly resistant to attack during later treatment steps. Furthermore, this approach does not address the root cause of pink ring and reason for multilayer board reject, namely fracture of board layers resulting from the stress of through-hole drilling.
Another approach employs reducing agents to reduce the copper oxide surface coating to cuprous oxide or metallic copper. However the reducing agents that are often employed, such as hydrazine or borohydrides, can be expensive as well as toxic, thus posing safety and disposal problems.